As a device for detecting an electric current of a three-phase inverter for controlling a three-phase brushless motor, a resistor voltage drop detector system is proposed. This system detects currents of the phases based on the voltage drops across the current detecting resistors interposed between low DC power source lines and lower-arm side switching transistors in the phases of the three-phase inverter, which is PWM-controlled.
According to the resistor voltage drop detector system, however, when the duty ratio of the lower-arm side switching transistor connected in series with the resistor assumes a small value of less than 30%, i.e., when the duty ratio of the upper-arm side switching transistor assumes a large value of more than 70%, the gate voltage waveform applied to the lower-arm side switching transistor is not often turned onto a sufficient degree. It thus becomes difficult to correctly detect the current of this phase.
In order to solve the above problem, a current detecting device is also proposed in JP-A-2003-164159. This device, when the duty ratio of the lower-arm side switching transistor of one phase assumes a small value making it difficult to detect an electric current, finds the electric current from the sum of currents of the remaining two phases by utilizing such a feature that the sum of currents of three phases (U-phase, V-phase and W-phase) of the brushless motor becomes 0 A.
According to the method of detecting an electric current as proposed in JP-A-2003-164159, abnormal condition cannot often be detected from the estimated electric currents flowing through the remaining two phases in case the duty ratio of the lower-arm side switching transistor of one phase assumes a small value as described above causing an abnormal current to flow through the one phase.
By providing an upper-arm side current detector circuit between the power source and the upper-arm side switching transistor, therefore, it becomes possible to detect an abnormal current flowing when the brushless motor is grounded in a region (duty) where the current cannot be detected by the above resistor voltage drop detector system.
In case the upper-arm side current detector circuit becomes abnormal, however, it becomes quite difficult to detect the abnormal current, and the motor and the peripheral equipment undergo malfunctioning.